Preparation Of Modified Wood Cellulose-based And Biochar Adsorbents And Application In Cadmium Removal

Cellulose comes from a wide range of sources;and is biodegradable and cost-effective.Through the modification of cellulose fiber,such as hemlock wood base-cellulose,the porous cellulose could act as a bioadsorbent.On the other hand,cellulose chips or powder can be carbonized via the pyrolysis at high temperature,thus creating biochars or activated carbon.Both cellulose fibres and biochar could be further loaded with metal oxides to improve their adsorption towards heavy metal ions,cadmium in particular,from industrial wastewater or stabilize cadmium pollution in soil.The implementation of biomass like cellulose or agricultural waste back into the field will provide green environmental protection and eliminate the secondary pollution.In this Thesis work,using cellulose as raw materials,biochar or woodiness active carbon was prepared via high temperature activation process,i.e.,pyrolysis,followed by loading with various metal oxides to create three kinds of cellulose-based adsorbents,i.e.,manganese oxide(CB-Mn),manganese aluminum double metal oxide(CE-Mn Al)and magnesium oxide(CB-Mg)-loaded cellulose or biochars.The resulting adsorbents were systematically characterized with FT-IR,SEM,EDX mapping etc to reveal the morphological structure,element types and contents.,The adsorption experiments of cadmium nitrate solution under different conditions were conducted along with identifying the effect of the adsorbents on the stability of cadmium ions in soil.The adsorption kinetics and isothermal adsorption model were fitted for each adsorbent in aqueous systems.The results indicated that Mn O2-loaded activated carbon adsorbent(CB-Mn)was modified successfully,and showed improved adsorption capacity towards cadmium ions in aqueous solution.With the solution p H 5,adsorbent additive amount of 2 g/L,the adsorption temperature at 25 ℃,and the initial concentration of cadmium nitrate solution at 50 mg/L,the adsorption efficiency reached about 99.4%,and the saturated adsorption capacity of 52 mg/kg.Through fitting different adsorption models,it was determined that the adsorption of Cd on CB-Mn was more in line with the quasi-secondary adsorption kinetic model,indicating that it was mainly chemical adsorption.The adsorption isotherm data of CB-Mn showed that the entire adsorption process could be well described with Langmuir model,implying that monolayer adsorption is dominant.CB-Mn also showed the fixation effect on cadmium ions in the soil,with a cadmium removal efficiency of 18.75%,thus controlling the enrichment of cadmium by plants and reducing the harm brought to humans by the influence of food chain.The structure and adsorption performance Mn-Al bimetallic oxide-loaded adsorbent(CE-Mn Al)were also well characterized.The results indicated that acid-tolerant CE-Mn Al had an excellent adsorption capacity towards Cd ions.Under the conditions of p H at 3-7,the adsorbent dosage 2g/L,the adsorption temperature25℃,and cadmium nitrate solution at 50mg/L,the adsorption efficiency reached up to 99%.The adsorption proceeded fast,reaching the adsorption equilibrium at 30min;and the maximum adsorption capacity was 76mg/g.By fitting different adsorption models,it was found that the adsorption of Cd on CE-Mn Al was more in line with the quasi-secondary adsorption kinetics model,further demonstrating the adsorption is it was mainly chemical-governed.Different from CB-Mn,however,the adsorption isotherms of CE-Mn Al showed that the adsorption was more consistent with Freundlich model,which suggested that the multilayer adsorption was dominant.Moreover,the removal efficiency of cadmium induced by CE-Mn Al was 15.92% for soil remediation when the effective cadmium content in soil was 13.13mg/kg.The results showed that the adsorbent had a relatively strong fixation effect on the cadmium in the soil,promoted the transformation of the free cadmium ions in the soil to the oxidation state and the residue state,thus reducing the migration and bioavailability of the cadmium ions.The magnesium metal oxide-supported wood activated carbon(CB-Mg)was prepared via the pyrolysis in a muffle furnace and characterized with EDX mapping and SEM.The experimental data showed that CB-Mg has excellent acid resistance and high adsorption capacity towards Cd ions(up to 163.29 mg/g)under the conditions similar to those for CE-Mn Al system.However,the adsorption proceeded even faster,and the adsorption equilibrium was reached in 5mins.The adsorption was also in line with the quasi-secondary adsorption kinetics mode and mainly chemical-controlled.Similar to CE-Mn Al,the adsorption isotherm for CB-Mg was more consistent with the Freundlich model,and the multilayer adsorption was dominant.For the stabilizing effect in the soil with an effective cadmium content of13.13 Mg /kg,the cadmium removal efficiency was 31.99% when CB-Mg was used alone;and 58.34% when mixed with calcium carbonate,which was the highest among three types of adsorbents.